Rename PyUnicode_AsString -> _PyUnicode_AsString and
PyUnicode_AsStringAndSize -> _PyUnicode_AsStringAndSize to mark
them for interpreter internal use only.

We'll have to rework these APIs or create new ones for the
purpose of accessing the UTF-8 representation of Unicode objects
for 3.1.
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 4e36dac..3bbc7e4 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -1424,7 +1424,7 @@
 		return 1;
 	}
         if (PyUnicode_Check(in)) {
-		*out = PyUnicode_AsString(in);
+		*out = _PyUnicode_AsString(in);
 		return 1;
 	}
 	if (PyTclObject_Check(in)) {